Apple Inc. is expected to report Tuesday that its stockpile of cash has topped a quarter of a trillion dollars, an unrivaled hoard that is greater than the market value of either Wal-Mart Stores Inc. or Procter & Gamble Co. and exceeds the foreign-currency reserves held by the U.K. and Canada combined.

The money, more than 90% of which is stockpiled outside of the U.S., has drawn fresh attention as President Donald Trump has proposed slashing business taxes and granting a one-time tax holiday on corporate cash brought...